Ducted fan gas turbine engines for aircraft propulsion normally comprise a core engine which drives a propulsive fan. The fan in turn comprises a number of radially extending aerofoil blades mounted on a common hub and enclosed within a generally cylindrical casing.
There is a remote possibility with such engines that part or all of one or more of the fan blades could become detached from the remainder of the fan. This might be as the result of, for instance, the engine ingesting a large foreign body, such as a bird. In the event of this happening, it is extremely important that the detached blade or blade portion is contained by the structure which surrounds it. Conventionally this means that the fan casing which surrounds the fan blades must be sufficiently strong to ensure that the detached blade or blade portion does not pass through the casing and cause damage to the aircraft carrying the engine.
There are various ways in which the problem of fan blade containment may be tackled. The most obvious way is to manufacture the fan casing from an alloy which is sufficiently strong and thick to provide the desired degree of containment. However this almost invariably results in a fan casing which is undesirably heavy. An alternative approach is to provide an aluminium alloy fan casing which is thin, and therefore light, and wind around it a strong fibrous material such as a fibrous aromatic polyamide. In the event of all or part of a fan blade becoming detached, the detached portion passes through the thin aluminium alloy casing but is contained by the fibrous material. Such a containment system is described in GB2159886B.
While the use of a fibrous containment material in this way is effective in providing fan blade containment, there are certain drawbacks to its use. Since the fibrous containment material is positioned close to the air which is compressed by the fan, it tends to heat up. Certain strong, lightweight fibrous containment materials which would otherwise be well suited to use in such an application cannot be so used in view of their poor tolerance to high temperatures in this sort of environment. Consequently high temperature tolerant, but unfortunately heavy, fibrous containment material must be used, thereby having a detrimental effect upon overall engine weight.
A further drawback is that in the event of a fan blade failure and subsequent successful containment, there can be rubbing between what remains of the fan and its surrounding casing. Such rubbing can result from rotation of the fan through continuing engine operation or through the air flow over the fan resulting from aircraft motion. This can lead in turn to undesirable overheating of the fan and casing. Additionally during containment, significant additional axial loads are placed upon the fan blades. Consequently strong, and usually undesirably heavy locking devices have to be applied to the blade roots to prevent further fan blades being lost.
A yet further drawback is that conventionally, the intake portion of the nacelle which encloses the engine is supported by the fan casing. If the fan casing is destroyed, such support is lost, thereby possibly causing loss of the nacelle intake portion.